This invention relates to a process for making 2-aryl benz(ox, thi, imid)azoles and 2-aminoaryl amino-benz(ox, thi, imid)azoles. The term "benz(ox, thi, imid)azole" is used herein as a shorthand term to designate an oxazole, thiazole or imidazole group which is fused to an aromatic ring at the 4 and 5 positions.
Polyimide benz(ox, thi, imid)azole (PIBX) polymers can be prepared by reacting a dianhydride with a diamine containing one or more benz(ox, thi, imid)azole groups. See, for example U.S. Pat. No. 4,087,409 to Preston, incorporated herein by reference. Among the useful benz(ox, thi, imid)azole-containing diamines are those in which the 2 position of the oxazole ring is substituted with an aminoaryl group. Among the diamines of the latter type are those represented by the structure: ##STR1##
wherein X is --O--, --S--, or --NH--.
These latter amines can be prepared by reacting an amino benzoic acid with 2,4- or 2,5-diamino phenol, 2,4- or 2,5-diamino phenyl mercaptan, or 1,2,4-triaminobenzene (when X is --O--, --S--, and --NH--, respectively). For example, it is known to prepare 2-(m-aminophenyl)-aminobenzoxazole (DAMBO) by reacting 2,4-diaminophenol with meta- or para-aminobenzoic acid in the presence of polyphosphoric acid. Unfortunately, however, this process suffers from several drawbacks. The reaction mixture requires extensive neutralization, and thus forms large volumes of aqueous phosphate salts as a waste stream. The product must be purified extensively through repeated sublimations in order to be useful as a monomer. In addition, the diaminophenol and aminobenzoic acid starting materials are expensive and not readily available.
For these reasons, it would be desirable to provide an alternate method for making DAMBO as well as other 2-aminoaryl aminobenz(ox, thi, imid)azoles.